Apparatus and method for synchronizing channels in a WCDMA communication system

ABSTRACT

A synchronization (sync) code communication device for a CDMA communication system. A base station sync code transmission device comprises a primary sync code transmitter and a secondary sync code transmitter. The primary sync code transmitter is for generating and then transmitting a primary sync code at a first location in a transmission frame. The primary sync code is for acquiring synchronization at a starting point of a frame and the frame is equal to one period of a spreading code. The secondary sync code transmitter is for generating and then transmitting a secondary sync code at a second location in a frame. The secondary sync codes are assigned to base station groups, one to a group. A mobile station sync code receiving device comprises a primary sync code acquisition decider and a base station group decider. The primary sync code acquisition decider is for acquiring a primary sync code received at a first location in a frame, and then acquiring synchronization at a starting point of a frame. The base station group decider is enabled upon acquisition of the primary sync code, receives a secondary sync code transmitted at a second location in the frame, and then determines the base station group to which the transmitting base station belongs.

PRIORITY

This application claims priority to an application entitled “Apparatusand Method for Synchronizing Channels in W-CDMA Communication System”filed in the Korean Industrial Property Office on Apr. 29, 1999 andassigned Serial No. 99-15332, and an application filed in the KoreanIndustrial Property Office on May 25, 1999, and assigned Serial No.99-18921, the contents of both of which are hereby incorporated byreference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to a synchronizing device andmethod for a CDMA (Code Division Multiple Access) communication system,and in particular, to a device and method for synchronizing channels ina W-CDMA (Wideband-CDMA) communication system.

2. Description of the Related Art

Next generation W-CDMA mobile communication systems assign unique basestation codes to each base station to perform asynchronous operationbetween the base stations. For 512 cells, 512 unique codes are assignedto identify 512 base stations. In such an asynchronous mode base stationcommunication system, a mobile station detects the base station signalbeing currently received at the highest power, in order to successfullyperform a call. However, in the asynchronous base station system, ittakes quite a long time to examine the phases of all the possible codesin the cell search, so that it is difficult to apply a general cellsearch algorithm. Therefore, a multi-step cell search algorithm has beenproposed. This method classifies 512 cells into 32 groups and each groupincludes 16 cells. To employ this method, sync channels are used whichinclude a primary sync channel signal (code) and a secondary syncchannel signal (code).

FIG. 1 shows a sync channel structure used for cell search in anasynchronous W-CDMA system. In FIG. 1, reference numeral 1-1 denotes aprimary sync channel (PRIMARY SCH) signal, reference numeral 1-3 denotesa secondary sync channel (SECONDARY SCH) signal, and reference numeral1-5 denotes a common pilot channel signal. One frame has 16 slots. Theprimary sync channel signal and the secondary sync channel signal aretransmitted for a N-chip (256 chip) length at the starting point ofevery slot. Orthogonality between the two channel signals is maintainedso that they can be transmitted at the same time. Further, the commonpilot channel uses a unique PN (Pseudo Noise) code (spreading code) foreach base station, and the period of the PN code is identical toone-frame length.

The W-CDMA system having the above channel structure uses Gold codes ofperiod 2¹⁸−1 for the unique PN codes, and uses only M (=512) codes outof all possible Gold codes of that length. The common pilot channelsignal is not transmitted simultaneously with the primary sync channelsignal and the secondary sync channel signal, but only transmitted atother time periods.

The sync channels use sync codes, and the sync codes are generated byperforming modulo operation between a Hadamard sequence and ahierarchical sequence. The hierarchical sequence y is generated using asequence x₁ of length n₁ and a sequence x₂ of length n₂, as follows:y(i)=x₂(imodn₂)+x₁(i+n₁) for i=0, . . . , (n₁*n₂)−1

Further, the sequences x₁ and x₂ are select sequences of length 16 asfollows.x₁=<0,0,1,1,0,1,0,1,1,1,1,1,0,0,0,1>x₂=<0,0,1,1,1,1,0,1,0,0,1,0,0,0,1,0>

The Hadamard sequences are obtained as the rows in a matrix H₈constructed recursively by:

${H_{k} = \begin{pmatrix}H_{k - 1} & H_{k - 1} \\H_{k - 2} & H_{k - 1}\end{pmatrix}},{k \geq 1}$

The rows are numbered from top starting with row 0 (the all onessequence). The nth Hadamard sequence is denoted as the nth row of H₈numbered from the top, n=0,1,2, . . . , 255, in the sequel.

Therefore, let h_(m)(i) and y(i) denote the ith symbol of the sequenceh_(n) and y, respectively where i=0, 1, 2 . . . , 255 and i=0corresponds to the leftmost symbol.

By XOR-gating a 256-chip Hadamard sequence h_(m)(i) and the hierarchicalsequence y(i), a kth sync code is then defined asC_(sc. k)={h_(m)(0)+y(0), h_(m)(1)+y(1), h_(m)(2)+y(2), . . . ,h_(m)(255)+y(255)},

Where m=8×k, k=0,1,2, . . . , 17, and the leftmost chip in the sequencecorresponds to the chip transmitted first in time.

Then, synchronization code #0 generated in the above manner is assignedto the P-SCH signal, whereC_(p)=C_(sc. 0)

The other synchronization codes, C_(sc. 1) to C_(sc, 17) are assigned inthe respective slots of a secondary sync (S-SCH) signal.

The primary sync code c_(p) is repeatedly transmitted only for 256 chipsevery slot, which is 1/10 of one slot. The sync code used for theprimary sync channel signal is the same for every cell. The primary syncchannel signal is used for detecting the slot timing of the receivedsignal by the mobile station. The base station transmitter introduces acomma-free code when transmitting the secondary sync channel. Thecomma-free code is comprised of 32 code words, and each code word iscomprised of 16 symbols and transmitted repeatedly in every frame.However, the 16 symbol values are not transmitted as they are, but eachsymbol value is mapped into a secondary sync code and is transmitted forframe synchronization and base station group information. The mobilestations have the comma free code table and know the mapping relation ofthe symbols and secondary sync codes. As shown in FIG. 1, an ith synccode, corresponding to a symbol value ‘i’, is transmitted every slot.C_(s) ^(i,k) indicates the ith secondary synchronization code insertedin the kth slot. The 32 code words of the comma-free code identify 32groups, and the comma-free code has a unique cyclic shift feature foreach code word. Therefore, it is possible to obtain information aboutthe code groups and frame synchronization using the secondary syncchannel signal (code). Here, “frame synchronization” refers tosynchronization of timing or phase within one period of a PN spreadingcode in a spreading spectrum system. However, in the existing W-CDMAsystem, since both one period of the spreading code and the frame lengthare equal to 10 ms, this PN code synchronization will be referred to asframe synchronization.

In the mobile station, a correlation value is calculated for a spreadingcode of a base station in order to distinguish different base stationcodes used by different base stations. Forward common channels, such asa pilot channel and a broadcasting channel (BCH), can be used whencalculating the correlation value for the spreading code of the basestation. In the conventional W-CDMA system, the pilot symbol istransmitted on the broadcasting channel using Time Division Multiplexing(TDM). However, the recent harmonization group OHG (OrganizedHarmonization Group) recommends transmitting the forward common pilot.FIG. 1 shows an example where the forward common pilot channel istransmitted by CDM (Code Division Multiplexing) and transmission of thepilot channel is discontinued when the sync code is transmitted.

FIG. 2 shows an example where the forward common pilot channel signal istransmitted by CDM and the pilot channel signal is continuouslytransmitted without discontinuation even when the sync channel signalsare transmitted.

The common pilot channel signal can transmit the pilot symbol and datausing time division multiplexing in every slot (the existing W-CDMAstructure). Otherwise, there may be provided separate channels fortransmitting the data. In this case, the channel frame for transmittingdata should have the same boundary as the common pilot channel frame.Generally, the common pilot channel does not transmit data, but onlytransmits the pilot symbol, all +1 or −1.

In the synchronization process of the conventional W-CDMA system, thesynchronization is acquired through three search steps. In the firststep, synchronization of the 0.625 ms slot is acquired. In the secondstep, frame synchronization is acquired and group identification isperformed. In the third step, the spreading code (the specific basestation code) used in the group is decided.

However, in the conventional synchronization process, when performingthe frame synchronization and group identification of the second step,the secondary sync channel is undesirably monitored for a period of 10ms. That is, in a conventional CDMA communication system, it is notpossible to acquire frame synchronization within one period of thespreading code. Further, in the W-CDMA communication system, it is notpossible to perform synchronous communication using only one syncchannel. Therefore, in a conventional CDMA communication system, thefrequent repetitions of transmitting the sync code makes it impossibleto minimize interference on the forward link. Accordingly, it is notpossible to increase the system capacity.

In addition, in the conventional system, in order to synchronizeinformation about a code group with a frame, the secondary sync channelmust be received continuously during one frame. The present inventionaims at reducing the overall synchronization time by minimizing the timerequired for receiving the secondary sync channel.

SUMMARY OF THE INVENTION

It is, therefore, an object of the present invention to provide a deviceand method for minimizing communication of the sync channel signal whileacquiring synchronization in a W-CDMA communication system.

It is another object of the present invention to provide asynchronization device and method for a base station in a W-CDMAcommunication system, wherein a primary sync code for framesynchronization is transmitted at a predetermined location within a oneframe period, and a secondary sync code corresponding to the code groupto which the base station belongs is transmitted at a locationpredetermined chip size distance from the primary sync code.

It is further another object of the present invention to provide asynchronization device and method for a base station in a W-CDMAcommunication system, wherein a primary sync code for framesynchronization is transmitted at a predetermined location within a oneframe period, and a secondary sync code corresponding to the code groupto which the base station belongs, is transmitted at a previously setlocation after the primary sync code.

It is yet another object of the present invention to provide asynchronization device and method for a base station in a W-CDMAcommunication system having a plurality of antennas which support atransmit diversity function, wherein a primary sync code for framesynchronization is transmitted through the antennas at a predeterminedlocation within a one frame period, and a secondary sync codecorresponding to the code group, to which the base station belongs, istransmitted at a previously set location after the primary sync code.

It is still another object of the present invention to provide asynchronization, device and method for a base station in a W-CDMAcommunication system, wherein every base station uses the same spreadingcode instead of a secondary sync channel code, and each base stationtransmits a primary sync channel code at a predetermined offset locationwithin a frame length.

It is yet still another object of the present invention to provide asynchronization device and method for a mobile station in a W-CDMAcommunication system having a base station which transmits a primarysync channel code for frame synchronization at a predetermined location,which is known at the mobile station by a predetermined mobilecommunication standard, within a one frame period and transmits asecondary sync channel code corresponding to the code group, to whichthe base station belongs, at a location a predetermined chip sizedistance from the primary sync channel code, wherein the synchronizationdevice and method determines whether frame synchronization is acquiredor not by acquiring the received primary sync channel code and thendetermines the secondary sync channel code, after acquisition of theprimary sync channel, to determine a code group.

In accordance with one aspect of the present invention, a sync channeltransmission device for a base station in an asynchronous CDMAcommunication system comprises a primary sync channel transmitter forgenerating a primary sync code to indicate the starting point of oneframe, said frame equaling one period of a spreading code of commonpilot channel, and transmitting the primary sync code at a firstlocation in the frame; and a secondary sync channel transmitter forgenerating a secondary sync code assigned to the group of base stationsto which the base station belongs, and transmitting the secondary synccode at a second location in the frame.

In accordance with another aspect of the present invention, a syncchannel receiving device for a mobile station in an asynchronous W-CDMAsystem comprises a primary sync channel acquisition decider foracquiring a primary sync channel code received at a first location in aframe, and acquiring synchronization for the starting point of a frame,said frame equaling one period of a spreading code of common pilotchannel; and a code group decider receives a secondary sync channel codetransmitted at a second location in the frame, and determining the basestation group to which the corresponding base station belongs by thedistance of the primary synchronization code and secondarysynchronization.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other objects, features and advantages of the presentinvention will become more apparent from the following detaileddescription when taken in conjunction with the accompanying drawings inwhich:

FIG. 1 is a diagram illustrating a sync channel structure of aconventional W-CDMA communication system;

FIG. 2 is a diagram illustrating another sync channel structure of aconventional W-CDMA communication system;

FIGS. 3A to 3C are diagrams illustrating several methods for generatingone sync channel within one period of a spreading code according to anembodiment of the present invention;

FIG. 4 is a diagram illustrating a sync channel structure according to afirst embodiment of the present invention;

FIG. 5 is a diagram illustrating a sync channel structure according to asecond embodiment of the present invention;

FIG. 6 is a diagram illustrating a time slot and sync code assignmenttable used in the sync channel structure according to the secondembodiment of the present invention;

FIG. 7A is a diagram illustrating a sync channel structure according toa third embodiment of the present invention;

FIG. 7B is a diagram illustrating a sync channel structure according toa fourth embodiment of the present invention;

FIG. 8A is a diagram illustrating a sync channel structure according toan embodiment of the present invention in a CDMA communication systemsupporting antenna diversity;

FIG. 8B is a diagram illustrating a sync channel structure according toanother embodiment of the present invention in a CDMA communicationsystem supporting antenna diversity;

FIG. 9A is a diagram illustrating a structure of a common pilot channeland a sync channel according to an embodiment of the present invention;

FIG. 9B is a diagram illustrating a structure of a common pilot channeland a sync channel according to another embodiment of the presentinvention;

FIG. 10A is a diagram illustrating a case where a sync channel and apilot channel use different long codes according to an embodiment of thepresent invention;

FIG. 10B is a diagram illustrating a case where the sync channel and thepilot channel use the same long code according to an embodiment of thepresent invention;

FIG. 10C is a diagram illustrating a case where every base station usesthe same long code for the sync channels and different long codes forthe pilot channels, and the same long code is used on a group unit basisaccording to an embodiment of the present invention;

FIG. 11A is a diagram illustrating a structure of a channel transmitterfor transmitting sync channel codes according to an embodiment of thepresent invention;

FIG. 11B is a diagram illustrating a structure of a channel transmitterfor transmitting sync channel codes in a CDMA communication systemsupporting antenna diversity according to an embodiment of the presentinvention;

FIG. 12 is a diagram illustrating a receiving device for a mobilestation in a CDMA communication system having the sync channel structureaccording to an embodiment of the present invention;

FIG. 13 is a flow chart illustrating an operation performed in theprimary sync channel acquisition decider of FIG. 12 according to anembodiment of the present invention;

FIG.14 is a diagram illustrating another receiving device for a mobilestation in a CDMA communication system having the sync channel structureaccording to an embodiment of the present invention;

FIG. 15 is a diagram illustrating the despreader and the code groupdecider of FIG. 14 according to an embodiment of the present invention;

FIG. 16 is a flow chart illustrating an operation performed in thedecider of FIG. 15 according to an embodiment of the present invention;

FIG. 17A is a diagram illustrating a scheme for generating a sync codefor an asynchronous primary sync channel in a CDMA communication systemaccording to an embodiment of the present invention;

FIG. 17B is a diagram illustrating a scheme for generating a sync codefor a synchronous primary sync channel in a CDMA communication systemaccording to an embodiment of the present invention;

FIG. 18 is a flow chart illustrating a procedure for determining, in amobile station, a base station code by analyzing sync codes transmittedfrom an asynchronous or synchronous system according to an embodiment ofthe present invention; and

FIGS. 19A to 19C are diagrams illustrating information field structuresfor performing neighbor cell search in a CDMA communication systemaccording to an embodiment of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

A preferred embodiment of the present invention will be described hereinbelow with reference to the accompanying drawings. In the followingdescription, well-known functions or constructions are not described indetail since they would obscure the invention in unnecessary detail.

In the following description, every base station shares the same codefor the primary sync code transmitted over a primary sync channel. inaddition, the secondary sync code transmitted over the secondary syncchannel indicates a code group of the base stations.

The present invention relates to initial synchronization in a CDMAcommunication system. As in the conventional W-CDMA system, the basestations are identified by unique spreading codes, which are classifiedinto several groups. For example, a base station can spread a forwardlink using 512 unique spreading codes, which can be classified into 32groups. Thus, each group includes 16 spreading codes. A mobile stationperforms initial acquisition and cell search without knowing the timesync or which spreading code the base station is presently using.Moreover, it is very difficult for the mobile station to acquire initialacquisition because the mobile station must test all the possiblehypotheses without having either the spreading code information or theinitial time sync information. Therefore, in this situation, there is aneed for the mobile station to effectively acquire initial acquisition.

The present invention proposes a method which acquires synchronizationat a boundary of one period of a frame which is one spreading codeperiod using at least one primary sync channel code inserted everyperiod of the frame, receives the primary sync channel code every periodof the frame, and thereafter detects at least one secondary sync channelcode received with or without a time delay, thereby identifying thegroup to which the base station belongs based on the secondary syncchannel code. The secondary sync channel code is unique to all the basestations in a base station group. Therefore, the mobile station canidentify the base station group by detecting the secondary sync channelcode. Further, the invention proposes a sync channel structure which caneffectively perform frame synchronization and group identification of aspreading sequence. In the following W-CDMA system examples, one periodof a spreading code used in the forward link is identical to the framelength. Herein, “frame synchronization” refers to acquiringsynchronization for transmission timing within one period of thespreading code.

Further, the present invention proposes a scheme in which a code for theprimary sync channel is transmitted one or more times every period orframe of the spreading code, and a code for the secondary sync channelis transmitted either at the same time as the code for the primary syncchannel or with a time delay after transmission of the code for theprimary sync channel. Here, in order to enable the receiver to easilyacquire the transmitted sync channel codes, the primary sync channelsignal (code) is transmitted as the PN code which is commonly used byevery base station, and the secondary sync channel signal (code) istransmitted as the spreading sequence or a code for groupidentification. The secondary sync channel code is unique for all thebase stations in a base station group.

The receiver attempts to acquire the primary sync channel code, and whenthe mobile station succeeds in acquisition of the primary sync channelcode, synchronization is acquired for the boundary of the spreading code(i.e., a boundary of the frame). At this point, we assume the basestation transmits the primary sync code at the starting point of theframe or with a predetermined time delay, which is previously determinedfor the entire system, so that it is known by all base stations and theall mobile stations.

Thereafter, it is necessary for the mobile station to detect the groupto which the transmitting base station belongs, and the spreading codewhich is being used for spreading pilot channel signal. The mobilestation distinguishes the group of the base station by detecting thesecondary sync channel code. For the secondary sync channel, each grouphas a unique code. For group identification, the codes used for thesecondary sync channel of each base station group can be eitherorthogonal or not orthogonal with each other. It is possible to simplyimplement the receiver using fast Hadamard transform (FHT), whilesecuring an orthogonality as between the codes. The receiver despreads areceived signal including secondary sync codes on the secondary syncchannel and selects the secondary sync code having the highest energy(i.e., a group having a higher probability), and then decides theselected group is the group to which the transmitting base stationbelongs. After finding out which secondary sync code is being used bythe transmitting base station, the mobile station performs despreadingon all the possible spreading codes of the group to which the basestation belongs, and selects a spreading code having the highestprobability according to the despreading results. Here, it is possibleto detect at least one of the spreading code used in one group throughthe forward common channels such as the pilot channel or thebroadcasting channel.

FIGS. 3A to 3C show a sync channel structure for frame synchronizationaccording to an embodiment of the present invention. Referring to FIGS.3A to 3C, “frame synchronization” refers to a procedure for acquiringtiming synchronization within one period of a spreading code in a spreadspectrum system. FIGS. 3A to 3C show a case where a sync channel signalis transmitted at a specific location of a spreading code period in thespread spectrum system. A receiver corresponding to a transmitter havingthe above channel structure first acquires a sync channel signal andthereafter automatically acquires frame synchronization. Herein, “framesynchronization” refers to synchronization for initial timing or phasewithin one period of a PN spreading code in the spread spectrum system.However, in the existing W-CDMA system, since one period and the framelength of the spreading code are both equal to 10 ms, to find the starttime of 10 ms will be referred to as frame synchronization. At thispoint, the sync channel signal can be acquired using the conventionalmatching filter. When comparing this with the synchronization process inthe conventional W-CDMA, it is possible to acquire frame synchronizationin a single process, using one sync channel at a low frequency. In thiscontext, frequency refers to the number of transmissions, and lowfrequency means fewer repeated transmissions.

FIG. 3A shows a method for transmitting a sync channel at apredetermined location within one period of a spreading code in a spreadspectrum system using a spreading code of period P. Here, “predeterminedlocation” refers to a location a specific length L away from thestarting point (i.e., initial state) of one period of the spreadingcode, wherein the value L is predetermined value known by both thetransmission side and the receiving side. The sync channel signal istransmitted for a length of N chips, and it is assumed herein that thesync channel signal is transmitted for 256 chips. The receiver acquiresthe sync channel signal using the matching filter. After acquisition ofthe sync channel signal, the receiver can automatically acquiresynchronization for timing of the PN spreading code. That is, it isnoted that the starting point of one period of the spreading code (i.e.,a starting point of the frame) begins at L chips before the acquiredsync channel.

FIG. 3B shows a case where L=0. That is, shown is a case where astarting point of the sync channel signal is concurrent with a startingpoint of a period of the spreading code. FIG. 3C shows a case where forL=P−N, an ending point of the sync channel signal is concurrent with astarting point of a next period of the spreading code.

In the case where there is only one PN code used for a spreading code,completing acquisition of the sync channel signal is equivalent tocompleting acquisition of a spreading code. However, in the case wherethere are several PN codes used for the spreading code, each basestation having its own unique spreading code, acquisition for thespreading code is performed in two steps as follows. The receiver firstperforms acquisition on the sync channel. When timing for the syncchannel is acquired, the mobile station does not know which spreadingcode is used, but has already acquired information about a phase (ortiming) of the spreading code. The receiver calculates correlationvalues by performing despreading on all the possible spreading codesusing the timing information to detect the maximum value out of thecalculated correlation values, or to detect the used spreading code bycomparing the values with a threshold value or combining the values, soas to acquire final synchronization.

FIGS. 3A to 3C show the cases where the sync channel signal istransmitted only once in one period of the spreading code. However, itis also possible to acquire timing of the spreading code using the syncchannel even in the case where the sync channel signal is transmittedonly once over several periods of the spreading code. Alternatively, thesync channel signal can be transmitted several times within one periodof the spreading code.

FIG. 4 shows a sync channel structure according to a first embodiment ofthe present invention. Referring to FIG. 4, the base station transmits aprimary sync code and a secondary sync code for one period of PNsequence. The primary sync code has a length of N-chip period and thesecondary sync code has a length of N₂-chip period. In the embodimentsof the present invention described below, it will be assumed thatN₁=N₂=256 chips.

Referring to FIG. 4, the primary sync code is transmitted L₁ chips aftera boundary of the frame (i.e., a starting point of one frame) or after aboundary of one period of the spreading code. In some cases, the L₁value can be 0, resulting in the primary sync code being transmitted atthe boundary of the frame. In addition, the secondary sync code istransmitted L₂ chips after the ending point of the primary sync code.The secondary sync code and the group to whiCh the base station belongs,correspond to each other according to a predetermined mapping rule(i.e., first secondary code indicate first group). Therefore, when themobile station detects the secondary sync code, the mobile station canidentify the group to which the base station belongs.

In order to enable coherent demodulation by transmitting the primarysync code and the secondary sync code through the same antenna, it ispreferable to transmit the L₂-chip interval between the two sync codeswithin a coherent time. In FIG. 4, L₂ is the guard interval between theprimary sync code and the secondary sync code. In the first embodimentof the present invention, it is assumed that the interval L₂ between thetwo sync code is 256 chips. Here, it is also possible to set L₂=0 toconsecutively transmit the secondary sync code after transmission of theprimary sync code. However, when the mobile station attempts to detectthe secondary sync code immediately after detecting the primary synccode, it is possible to permit a slight time interval so that the mobilestation can detect the secondary sync code with a slight time delay. Adetailed description of this will be made later with reference to adescription of the receiver.

FIG. 5 shows a sync channel structure according to a second embodimentof the present invention. In the sync channel structure according to thefirst embodiment of FIG. 4, the secondary sync code is identified usingthe different spreading codes. However, in the sync channel structureaccording to the second embodiment of FIG. 5, groups (alphabets) of thesecondary sync codes can be given in combination of different startingtime. In addition, FIG. 6 shows alphabet allocation for the secondarysync code in the sync channel structure according to the secondembodiment.

Referring to FIG. 5, if the number of alphabets for the secondary synccode used for group identification of the base station is X, those areidentified in (m*n) combinations (X≦m*n) of (T₁−T_(m)) time slots and(C₁−C_(n)) different spreading codes. In FIG. 6, it is .assumed that thenumber of alphabets is 20, the number of used time slots is m=5, and thenumber of spreading codes used for one time slot is n=4. The term“alphabet” refers to the number of signals available for one symbol totransmit information about the code group or frame synchronization overthe secondary sync code. Alphabets corresponds to groups. In the presentinvention, it is assumed that each second sync code is transmitted byrepeating the code group information and the receiver can identify thecode group to which the base station belongs, even though the receiverreceives only a single code.

When a secondary sync code related to an alphabet is determined in theabove sync channel structure, the time slots in which to transmit, asindicated by the alphabet allocation for the sync channel, shown in FIG.6, and the spreading codes to be used at that time are determined. Aftertransmission of the primary sync code, the secondary sync code will betransmitted with a designated code at a designated time slot. Here, thesync codes used for one time slot can be orthogonal to each other. Inaddition, to avoid a collision between the signals from differentneighbor base stations (i.e., to avoid performance degradation due tocross-correlation coefficients), it is possible to have the groups ofthe codes used for the different time slots become exclusive to eachother. The reason for distributing transmission of the alphabets (codes)in the second dimension of time and code is because there are too manypossible codes for the secondary sync code to be despread at one timeslot. By distributing the alphabets in combination of the time slots andthe spreading codes, when the receiver performs simultaneousdespreading, the number of the codes to be tested is drasticallyreduced.

In FIG. 5, reference numeral 41 shows a case where a used alphabet (or agroup of the spreading code) is 3, and reference numeral 42 shows a casewhere a used alphabet (or a group of the spreading code) is 10. Here, ifthe alphabet to be transmitted over the secondary sync channel is 3 (inthis case, it is identical to an ID of the group), the secondary synccode transmitted by the base station is transmitted at the first timeslot according to alphabet assignment of FIG. 6 and at this time, thethird code C₃[i] is transmitted. Here, ‘i’ in the brackets indicatesthat the transmitted code can vary according to the time slot. That is,it means that the code to be transmitted can vary at each time slot.Thus, C₃[1] is not only transmitted at a different slot than C₃[2], butis also a different code. Further, when an alphabet of the secondarysync channel to be transmitted is 10 as shown by reference numeral 42,the third time slot and the second spreading code C₂[3] are assignedaccording to alphabet allocation of FIG. 6.

Therefore, when the base station transmits the secondary sync channel,which is C₂[3], at the third time slot to transmit the alphabet 10 asshown by reference numeral 42 of FIG. 5, the mobile station can find outthe information about the code group to which the base station belongs,by detecting C₂ at the third time slot. Thereafter, the mobile stationshould detect which spreading codes are used in the group to which thebase station belongs. To this end, the mobile station performsdespreading on all the possible spreading codes of the group to whichthe base station belongs and selects the spreading code having thehighest possibility. At this point, the mobile station can use theforward common channels such as the pilot channel and the broadcastingchannel in order to determine the used spreading code in one group. Inthe embodiment of FIG. 5, there exists an interval of L₃ chips betweenthe time slots. This is to spare a time to the receiver in demodulatingthe next slot after demodulating a signal on the previous slot using thesame hardware. However, the interval L₃ between the time slots can beset to 0, or the time slots can be overlapped. Of course, in this case,the receiver will be more complex in structure (i.e., have twodemodulators), as compared with the case where there exists an intervalbetween the time slots.

In the embodiments of FIGS. 4 and 5, the base station transmits theprimary sync channel once in one period or one-frame duration of thespreading code for the forward link, and the mobile station acquiressynchronization for this to acquire timing or frame synchronization ofthe spreading code. Thereafter, the code group is determined through thesecondary sync channel. The secondary sync channel can be transmitted atleast once. Transmitting more than once provides time diversity, and,thus, provides more reliability when receiving the secondary syncchannel. In particular, when the base station transmits the forward linkusing two or more antennas, it is possible to obtain an antennadiversity effect by alternately transmitting the symbols through theantennas.

Unlike the embodiments stated above, the base station can transmit theprimary sync code two or more times in one-frame period. This is reducethe overall acquisition time by increasing a synchronization speed ofthe primary sync channel. That is, the primary sync code is transmittedNUM_PRI times every one period or one-frame length of the PN spreadingsequence, and only one of them is acquired to perform sync acquisition.However, at this point, it is not possible to acquire synchronization ofthe spreading sequence (or synchronization of the frame) at thelocations of the NUM_PRI primary sync channels. That is, acquisition offrame synchronization and group detection is performed by analyzinginformation about the secondary sync channel. In the third and fourthembodiments of the present invention, a description will be made withreference to a case where the number of alphabets required for thesecondary sync code is NUM_PRI*NUM_GROUP. Here, NUM_GROUP denotes thenumber of groups for classifying the spreading codes of the basestation. The reason that so many alphabets for the secondary sync codeare required is because it is necessary to identify the ID of the groupsand acquire synchronization of one frame out of NUM-PRI candidates. Theinvention is different from the existing W-CDMA sync channel in that onesymbol (the secondary sync channel is composed of one symbol) of oneframe is demodulated in the second step in order to acquiresynchronization of group and frame. However, the sync channel structureaccording to the present invention is advantageous against the existingW-CDMA sync channel structure in that it is possible to performdetermination of the group ID and acquisition of frame synchronizationeven if only one secondary sync channel is received.

FIG. 7A shows a sync channel structure according to a third embodimentof the present invention, wherein NUM_PRI is 2.

Referring to FIG. 7A, the base station transmits the primary syncchannel twice in one-frame period as shown by reference numeral 71 ofFIG. 7A. Here, it is assumed that a time interval between transmittingthe primary sync code is half of one period P (i.e., P/2) of thespreading sequence as shown by reference numeral 71, and the L₁ value is0. However, L₁ can have a specific value and the transmission intervalof the primary sync channel in the one-frame period can be set to avalue other than P/2. The base station transmits the secondary sync codeat a location being part by L₂ shown by reference numeral 72 of FIG. 7Aafter transmission of the primary sync code. The L₂ value can be 0, whennecessary.

In FIG. 7A, the secondary sync code can be located at the center of thesymbols of two consecutive primary sync codes. However, in theembodiment of the present invention, it is assumed that the secondarysync code is transmitted 256 chips after completing transmission of theprimary sync code (L₂=256). By doing so, the invention has the followingadvantages.

The reason that the base station consecutively transmits the primarysync code and the secondary sync code is because the base station cancoherently demodulate the secondary sync code by performing channelestimation using the primary sync code when detecting the secondary synccode after acquiring the primary sync code. Of course, when the receiverhas a great frequency offset, it is not possible to perform coherentdetection. However, when the frequency offset is smaller or when aninitial frequency offset can be reduced to some extent using anautomatic frequency controller (AFC), it is possible to perform coherentdetection. That is, when the frequency offset is great during, forexample, initial acquisition, coherent demodulation is unavailable.However, during neighbor cell search and finger allocation, coherentdemodulation is available. In addition, the L₂ value can be set higherthan 0. For example, it is possible to leave an interval of 256 chips(=1 symbol) when the primary sync code and the secondary sync code areconsecutively transmitted as stated above, it is possible not only toperform coherent demodulation but also to minimize a synchronizationtime by attempting to detect the secondary sync code immediately afteracquiring the primary sync code having high power.

A primary goal of the present invention is to set a time intervalbetween the primary sync code and the secondary sync code within acoherent time so that channel estimation is performed using the primarysync code to coherently demodulate the secondary sync code. Further, aslight time interval is permitted between the primary sync code and thesecondary sync code to acquire the secondary sync code immediately aftera slight delay for determination when the primary sync code is acquiredwith reliability. That is, it is possible to reduce the overall searchtime by minimizing the interval between the primary sync code and thesecondary sync code, considering a slight processing delay of the mobilestation.

The number of alphabets transmitted over the secondary sync channel isassumed to be NUM_PRI*NUM_GROUP. This is to acquire information aboutcode group and frame synchronization even if only one secondary synccode is received.

FIG. 7B shows a sync channel structure according to a fourth embodimentof the present invention. The sync channel structure of FIG. 7B employsthe method, shown in FIG. 6, of transmitting the secondary sync code byassigning the time slots and alphabets. Although the sync channelstructure shown in FIG. 7B is identical to that of FIG. 7A in that aplurality of the primary sync codes and the secondary sync codes aretransmitted in one-frame period, time intervals L_(2′) and L_(2″)between the primary sync code and the secondary sync code can bedifferent from each other. This is because the primary and secondarysync codes may transmit different alphabets to transmit different framesync information. An increase in the frequency NUM_PRI of transmittingthe primary sync code causes an increase in amount of the syncinformation of the frame to be transmitted to the secondary sync code,and when one secondary sync code code includes both the groupinformation and the frame sync information as in an embodiment of thepresent invention, the number of alphabets to be transmitted over thesecondary sync channel increases drastically. That is, when there exist32 groups and the primary sync code is transmitted four times inone-frame period, the number of alphabets to be transmitted AT thesecondary sync code increases to 128. Further, when the primary synccode is transmitted eight times in one-frame period, the number ofalphabets to be transmitted to the secondary sync code increases to 256.This method may increase complexity of the receiver.

When the alphabets of the secondary sync channel are transmitted byassigning time slots and spreading codes, the number of spreading codesto be simultaneously despread is decreased. Therefore, it is possible todecrease complexity of the mobile station. The time slots and spreadingcodes can be assigned such that the time slots should indicate syncinformation of the frame and the spreading code should indicateinformation about the group to which the base station belongs. On theother hand, the time slots and spreading codes can be assigned such thatthe time slots should indicate information about the group to which thebase station belongs and the spreading code should indicate syncinformation of the frame. Here, “sync information of the frame” meansinformation about a location where the previous primary sync code has aspecific offset value within one frame.

In the CDMA system, the base station can employ antenna diversity whichtransmits a signal using several antennas. Although FIGS. 8A and 8B showa case where the primary sync code and the secondary sync code aretransmitted twice in a one-frame period, the invention can be appliedeven to a case where the primary sync code and the secondary sync codeare transmitted more than twice in the one-frame period.

FIG. 8A shows a case where the sync channels of FIG. 7A are transmittedusing two antennas. Referring to FIG. 8A, the primary sync code and thesecondary sync code connected thereto are transmitted through the sameantenna. This is because when the primary sync code having a high levelis detected, it is guaranteed that the secondary sync code also has ahigh level. In addition, by doing so, it is possible to coherentlydemodulate the secondary sync code using the primary sync code as achannel estimator. As shown in FIG. 8A, a first antenna Ant1 transmitsthe primary sync code and the secondary sync code connected thereto, andafter a specific time lapse, a second antenna Ant2 transmits the nextprimary sync code and the secondary sync code connected thereto.

FIG. 8B shows a case where the sync channels of FIG. 7B are transmittedusing two antennas. Similarly to FIG. 8A, the primary sync code and thesecondary sync code connected thereto are transmitted through the sameantenna. A difference between FIG. 8A and 8B is that the intervalbetween the primary sync code and the secondary sync code transmittedthrough one antenna is different from the interval between the primarysync code and the secondary sync code transmitted through the otherantenna. This is so when the primary sync code having a high level isdetected, it is guaranteed that the secondary sync code also has a highlevel.

FIG. 9A shows structures of a sync channel and a common pilot channelaccording to an embodiment of the present invention. In FIG. 9A,reference numeral 91 denotes the primary sync code and reference numeral92 denotes the common pilot code. This embodiment of the presentinvention relates to operation in an asynchronous system, and in theembodiment of FIG. 9A, the number of different spreading codes used forcell identification is limited to 16. Therefore, it is assumed hereinthat the secondary sync code used for group identification is notnecessary. In addition, a frame starting point can be detected bytransmitting the primary sync code once every frame, instead oftransmitting the primary sync code once every slot. In this embodimentof the present invention, the primary sync code is transmitted at higherpower (S≧1) as compared with the primary sync code in the asynchronousmode. That is, in this embodiment of the present invention, transmissionpower of the sync code is higher than or equal to 1. A starting point ofthe primary sync code is L chips (L≧0) from a starting point of thecommon pilot channel frame, and the primary sync code is transmitted Ktimes (K≧1) by P chips every frame. FIG. 9A shows a case where L=0 andK=1.

The common pilot channel can use different spreading codes for differentbase stations, or can apply different PN offsets to the same spreadingcode. A generation method and a period of the spreading code are thesame as the asynchronous mode. However, in the synchronous mode statedabove, a spreading code, which is different from the spreading code usedin the asynchronous mode, should be used. In FIG. 9A, the common pilotchannel is not transmitted in a period where the primary sync code istransmitted.

FIG. 9B shows a case where the common pilot channel is continuouslytransmitted even in a period where the primary sync code is transmitted.A description of each reference numeral in FIG. 9B is identical to adescription of the corresponding reference numeral in FIG. 9A.

FIGS. 10A and 10B show a case where only one PN code is used for aspreading code and base station identification is performed using a PNoffset. To show an example where a channel is spread with a PN spreadingcode, this embodiment of the present invention assumes that the commonpilot channel is transmitted by code division multiplexing (CDM).However, it should be noted that the invention can be easily appliedregardless of the channel structure. In the proposed method, thesecondary sync code is not used, a starting point of the primary synccode is apart by L chips (L≧0) from a starting point of one period ofthe spreading code, and the primary sync code is transmitted K times(K≧1) by N chips every period of the PN spreading code. FIGS. 10A and10B show a case where K=1 and L=0. The base stations are distinguishedwith different PN offsets. It is assumed that a base station BS1 has aPN offset of ‘0’, a base station BS2 has a PN offset of ‘2’ and a basestation BS3 has a PN offset of ‘3’. Starting points where the basestations transmit their primary sync codes have a time difference by thePN offsets, so that it is possible to acquire timing information of thespreading code upon acquisition of the primary sync code, regardless ofthe PN offset values of the base stations. The common pilot channel canbe either continuously transmitted or not transmitted at the time wherethe primary sync code is transmitted. The sync channels of the W-CDMAsystem are transmitted while maintaining orthogonality with otherchannels for the forward link. However, in FIG. 10A, the primary synccode can maintain orthogonality with other channel signals. This ispossible by XORing the spreading code for the forward link and a Walshfunction, for the sync codes.

In the embodiment of FIG. 10B where only one PN code is used for thespreading code for the forward link in the synchronous mode, N chips inthe period being part by L chips from the starting point of the PN codefor the pilot channel can be used for the primary sync channel. FIG. 10Bshows a case where L=0. In this case, it can be considered that theprimary sync code is transmitted at increased power in the correspondingperiod of the common pilot channel, and this can be implemented by gaincontrolling.

FIG. 10C shows a case where the number of different PN codes used forthe spreading code on the forward link is higher than 1. A startingpoint of the primary sync code is apart by L chips (L≧0) from a startingpoint of one period of the spreading code, and the primary sync code istransmitted once by N chip length every frame. FIG. 10C shows a casewhere L=0. The base stations are identified using different spreadingcodes or different PN offsets for the same spreading code. In theembodiment of FIG. 10C, it is assumed that the base stations BS1, BS2and BS3 commonly use the first spreading code and have different PNoffsets of PN offset 1, PN offset 2 and PN offset 3, respectively. ThePN offset 1 is 0. Further, it is assumed that the base stations BS4, BS5and BS6 all use the Hth spreading code and have the different PN offsetsof PN offset 1, PN offset 2 and PN offset 3, respectively. That is, thePN offset values which can be given for different base stations can beequal to each other because the spreading codes will be different.Starting points where the base stations transmit their the primary synccodes have time differences by the corresponding PN offsets, so that itis possible to acquire timing information of the spreading code uponacquisition of the primary sync code regardless of the PN offset valuesof the base stations. After acquiring the timing information of thespreading code, despreading is performed on the different spreadingcodes to detect the used spreading code, thereby finally acquiringsynchronization.

In FIG. 10C, when NUM_OFFSET=32 and H (=NUM_PN)=16, i.e., when 16different PN codes are used and the PN codes are distinguished with 32different PN offsets, it is possible to simultaneously distinguish 512cells. In the case where the system has a transfer rate of 3.84 Mcps andthe frame length is 10 ms, if 32 PN offsets are given for one PN code, aunit of the PN offset is 1200 chips.

FIGS. 11A and 11B show channel transmitters of the base station fortransmitting the primary and secondary sync codes according to differentembodiments of the present invention. More specifically, FIG. 11A showsa channel transmitter for transmitting the sync codes using one antenna,and FIG. 11B shows a channel transmitter for transmitting the sync codesusing two antennas in an antenna diversity method. A description will bemade of a method for transmitting the sync codes using antennadiversity, with reference to FIG. 11B.

Referring to FIG. 11B, a serial-to-parallel (S/P) converter 1111aparallel-converts a received common pilot channel signal to betransmitted through a first antenna Ant1 into I and Q channel data.Multipliers 1112a and 1113a spread the separated I channel and Q channelcommon pilot data with a channel spreading code C_(ch), respectively.The channel spreading codes used in FIGS. 11A and 11B can be expressedin complex number. A phase shifter 1114a shifts a phase of the spread Qchannel data by 90°. An adder 1115a adds outputs of the multiplier 1112aand the phase shifter 1114a to generate a complex spread signal I+jQ.

Similarly, a serial-to-parallel (S/P) converter 1111b parallel-convertsa received common pilot channel signal to be transmitted through asecond antenna Ant2 into I and Q channel data Multipliers 1112b and1113b spread the separated I and Q channel common pilot data with achannel spreading code C_(ch), respectively. A phase shifter 1114bshifts a phase of the spread Q channel data by 90°. An adder 1115b addsoutputs of the multiplier 1112b and the phase shifter 1114b to generatea complex spread signal I+jQ.

A serial-to-parallel converter 1121a parallel-converts a receivedprimary sync channel (P-SCH) signal to be transmitted through the firstantenna Ant1 into I and Q channel data Multipliers 1122a and 1123aspread the primary sync channel data separated into the I and Q channelswith a channel spreading code C_(p), respectively. A phase shifter 1124ashifts a phase of the spread Q channel data by 90°. An adder 1125a addsoutputs of the multiplier 1122a and the phase shifter 1124a to generatea complex spread signal I+jQ. This signal is transmitted to the firstantenna Ant1.

Similarly, a serial-to-parallel converter 1121b parallel converts areceived primary sync channel (P-SCH) signal to be transmitted throughthe second antenna Ant2 into I and Q channel data. Multipliers 1122b and1123b spread the primary sync channel data separated into the I and Qchannels with a channel spreading code C_(p), respectively. A phaseshifter 1124b shifts a phase of the spread Q channel data by 90°. Anadder 1125b adds outputs of the multiplier 1122b and the phase shifter1124b to generate a complex spread signal I+jQ. This signal istransmitted to the second antenna Ant2.

A serial-to-parallel converter 1131a parallel-converts a receivedsecondary sync channel (S-SCH) signal to be transmitted through thefirst antenna Ant1 into I and Q channel data Multipliers 1132a and 1133aspread the secondary sync channel data separated into the I and Qchannels with a channel spreading code C_(Sch), respectively. A phaseshifter 1134a shifts a phase of the spread Q channel data by 90°. Anadder 1135a adds outputs of the multiplier 1132a and the phase shifter1134a to generate a complex spread signal I+jQ. This signal istransmitted to the first antenna Ant1.

Similarly, a serial-to-parallel converter 1131b parallel-converts areceived secondary sync channel (S-SCH) signal to be transmitted throughthe second antenna Ant2 into I and Q channel data Multipliers 1132b and1133b spread the secondary sync channel data separated into the I and Qchannels with a channel spreading code C_(sch), respectively. A phaseshifter 1134b shifts a phase of the spread Q channel data by 90°. Anadder 1135b adds outputs of the multiplier 1132b and the phase shifter1134b to generate a complex spread signal I=jQ. This signal istransmitted to the second antenna Ant2.

The channel transmitter can further include common channels or dedicatedchannels in addition to the common pilot channel and the primary andsecondary sync channels. For such forward channel transmitters, therecan be provided the transmitters for forward common channels and forwarddedicated channels.

A gain controller 1100 determines whether to gate the channel andcontrols transmission power of the signals to be transmitted through thefirst and second antennas Ant1 and Ant2. Adders 1160a adds the gaincontrolled channel signals output from gain controllers 1116a, 1126a and1136a, respectively. An adder 1160b adds the gain controlled channelsignals output from gain controllers 1116b, 1126b and 1136b,respectively. Baseband filter 1161a and 1163a filter baseband signalsout of the signals output from adder 1160a, and baseband filters 1161band 1163b filter baseband signals out of the signals output from adder1160b. Multipliers 1162a and 1164a multiply outputs of the associatedbaseband filters 1161a and 1163a by the corresponding carriers.Multipliers 1162b and 1164b multiply outputs of the associated basebandfilters 1161b and 1163b by the corresponding carriers. The outputs ofthe multipliers 1162a and 1164a are added by an adder 1165a andtransmitted to the first antenna Ant1. The outputs of the multipliers1162b and 1164b are added by an adder 1165b and transmitted to thesecond antenna Ant2.

The channel transmitter, shown in FIG. 11A, for transmitting the syncchannel signals through a single antenna has the same operation aseither half of the channel transmitter shown in FIG. 11B.

FIG. 12 shows a receiver for the sync channel structure according to thefirst and second embodiments shown in FIGS. 4 and 5.

With reference to FIG. 12, operation of the receiver for the mobilestation will be described. A matching filter 1211 attempts to acquirethe sync channels by match filtering the received sync channel signals.Upon receipt of the output of the matching filter 1211, a primary syncchannel acquisition decider 1213 determines whether the primary syncchannel is acquired or not. Operation of the primary sync channelacquisition decider 1213 is shown in FIG. 13.

Referring to FIG. 13, the primary sync channel acquisition decider 1213receives the output of the matching filter 1211 and calculates I²+Q² tocalculate energy, in step 1311. Thereafter, in step 1313, the primarysync channel acquisition decider 1213 compares the calculated energywith a threshold value to determine whether synchronization is acquiredor not. If the calculated energy value is smaller than or equal to thethreshold value TH1, a first search step is continuously performed onthe next PN offset. Otherwise, if the energy value is larger than thethreshold value TH1 in step 1313, it is determined in step 1315 whethera peak signal is acquired or not. To this end, it is first determinedwhether the energy value has ever previously exceeded the thresholdvalue. If the energy value has never before exceeded the thresholdvalue, a second search step is started for the PN offset. Otherwise, ifthe energy value has exceeded the threshold value before, the primarysync channel acquisition decider 1213 compares the previous maximumvalue with the presently detected energy in step 1317. If the presentlydetected energy value is higher than the previous maximum value in step1317, the current second search step is ended and the second search stepis performed on the new PN offset in step 1319. Otherwise, when theprevious maximum value is higher than the presently detected energyvalue in step 1317, the existing second search step is continuouslyperformed continuously.

In the embodiments of FIGS. 4 and 5, frame synchronization is acquiredafter completion of the first search step, so that only informationabout the group to which the base station belongs should be determinedin the second search step.

Upon receipt of the decision results and the frame sync information fromthe primary sync code acquisition decider 1213, a controller 1200enables a despreader bank 1215 to perform the second search step basedon the acquisition information of the primary sync channel. Here, if anorthogonal code is used for the secondary sync channel code, thedespreader bank 1215 used for the second search step can be implementedby fast Hadamard transform (FHT). A code group decider 1217 receivingthe output of the despreader bank 1215, determines a code group to whichthe base station belongs in the second search step. When transmittingthe sync channels signal having the structure of FIG. 4, the receiverdespreads the secondary sync code after a lapse of a specified time (L₂chips) from an acquisition point of the primary sync code. The receiverincludes the despreader bank 1215, which has as many despreaders as thepossible code groups. The code group decider 1217 receives the output ofthe despreader bank 1215 and then determines a code group indicated by aspreading code having the highest detected energy as a code group of thecorresponding base station. In addition, when transmitting the syncchannels signal having the structure of FIG. 6, the despreader banks1215 performs despreading on the spreading code available in thecorresponding time slot at every time slot beginning at the acquisitionstarting point of the primary sync channel, and the code group decider1217 determines the code group indicated by the spreading code havingthe highest value as the code group of the corresponding base station.Although it is possible to determine the code group by receiving onesecondary sync code in the second search step, when the receivingreliability for the secondary sync code is not high enough, thesecondary sync code can be repeated until the reliability increases tosome extent.

When the information about the code group to which the base stationbelongs is acquired through the second search step, the controller 1200enables the despreader bank 1219. The despreader bank 1219 despreads thepossible spreading sequences in the code group determined and providesthe results to a spreading sequence decider 1221, in the second searchstep. The spreading sequence decider 1221 then determines whichspreading sequence is used for the spreading sequence and also,determines whether sync acquisition is performed successfully. Theresults are provided to the controller 1200 to finally notify whethersync acquisition is performed successfully.

FIG. 14 shows a receiver for the sync channel structure shown in FIGS.7A and 7B according to an embodiment of the present invention. The samereceiver can be applied even to the case where the base stationtransmits a signal using two or more antennas as shown in FIGS. 8A and8B.

With reference to FIG. 14, operation of the receiver for the mobilestation will be described. A matching filter 1411 match-filters an inputsignal to attempt acquisition for the primary sync code and provides theresults to a primary sync code acquisition decider 1413. The primarysync code acquisition decider 1413 then determines whether the primarysync code is acquired or not. Operation of the primary sync codeacquisition decider 1413 is performed according to the procedure shownin FIG. 13.

When the primary sync code acquisition decider 1413 provides thecontroller 1400 with the primary sync code acquisition decision resultsby performing the procedure of FIG. 13, the controller 1400 enables adespreader bank 1415 to perform the second search step based on thedecision results. At this point, if orthogonal spreading codes are usedfor the secondary sync code, the despreader bank 1415 used in the secondsearch step can be implemented by fast Hadamard transform (FHT). In thesecond search step, the receiver acquires the code group to which thebase station belongs, and frame synchronization. For the sync channelstructure shown in FIG. 7A, the despreader bank 1415 performsdespreading on the secondary sync code after a lapse of a specified timeL₂ from an acquisition point of the primary sync code. Here, for thedespreader bank 1415, there are provided as many despreaders as(possible code group number)*(NUM_PRI). A frame offset and code groupdecider 1417 determines, as code group and frame sync information of thecorresponding base station, the code group indicated by the spreadingcode having the highest detected energy out of the outputs of thedespreader bank 1415 and frame boundary information.

In addition, for the sync channel structure shown in FIG. 7B, thedespreader bank 1415 performs despreading on the possible spreadingcodes in the corresponding time slot every time slot beginning at anacquisition starting point of the primary sync code, and the frameoffset and code group decider 1417 determines, as code group and framesync information of the corresponding base station, a code groupindicated by a spreading code having the highest value out of thedespread values and the frame boundary information. Although it ispossible to determine the code group by receiving one secondary synccode in the second search step, when the receiving reliability of thesecondary sync code is not high enough, the secondary sync code can berepeatedly transmitted in order to increase the reliability to someextent.

FIG. 15 shows the despreader bank 1415 and the frame offset and codegroup decider 1417 for performing the second search step, and FIG. 16shows operation of the frame offset and code group decider 1417.

With reference to FIGS. 15 and 16, operation of the frame offset andcode group decider 1417 will be described. The despreader bank 1415includes a despreader bank (or FHT) 1511 and a time controller 1513 forcontrolling the despreader bank 1511. The frame offset and code groupdecider 1417 includes a reliability calculator 1521 and a decider 1523.When the despreader bank 1511 is enabled by the time controller 1513, aspreading code used for the secondary sync code is despread by thedespreader bank 1511 and the reliability is calculated for everyhypothesis of code group and frame synchronization. When the primarysync code is transmitted only once as shown in FIGS. 4 and 5, thereliability is calculated every code group hypothesis. One of the easymethods for calculating the reliability is to use an energy value(I²+Q²) of the despread results. The reliability for each hypothesis isprovided to a decider 1523, which decides the reliability.

FIG. 16 shows operation of the decider 1523. Referring to FIG. 16, thedecider 1523 orderly arranges the reliabilities for every hypotheses instep 1611 to determine a hypothesis having the highest reliability and ahypothesis having the second highest reliability. Thereafter, in step1613, the decider 1523 calculates a metric difference between thehypothesis having the highest reliability and the hypothesis having thesecond highest reliability, to determine whether synchronization of thesecond search step is performed or not. When the difference between thetwo values is lower than or equal to a threshold value, the decider 1523continuously receives the next secondary sync code, considering that thereliability of the second search step is not high enough. Otherwise,when the metric difference is higher than the threshold value in step1613, the decider 1523 makes a decision on the code group and framesynchronization, because the reliability of the second search step ishigh enough. Upon receipt of the code group decision results, thecontroller 1400 performs a third search step to finally detect thespreading code used by the base station.

After acquiring the code group, to which the base station belongs, andframe synchronization by performing the second search step, thecontroller 1400 enables a despreader bank 1419. The despreader bank 1419then performs despreading on the possible spreading sequences in thecode group decided in the second search step, and the results areprovided to a spreading sequence decider 1421. The spreading sequencedecider 1421 then determines which spreading sequence out of the outputsof the despreader bank 1419 is used for the spreading code and alsodetermines whether sync acquisition is performed successfully. Theresults are provided to the controller 1400 to finally notify whethersync acquisition is performed successfully.

In addition, the present invention provides a method for acquiring framesynchronization by transmitting one sync channel. The invention is moreeffective especially when the base stations operate in sync with aglobal positioning system (GPS). However, in the CDMA system, the basestations can operate in either in a synchronous mode or an asynchronousmode. The present invention provides a method for distinguishing a basestation system operating in sync with the GPS from a base station systemoperating out of sync with the GPS. That is, the invention distinguishesthe synchronous system from the asynchronous system by using differentsync sequences for the sync channels in the synchronous mode and theasynchronous mode. The reason for using the different sync sequences inthe synchronous mode and the asynchronous mode is to enable the mobilestation to rapidly determine to which system (synchronous system orasynchronous system) the mobile station itself belongs and to usedifferent sync channels in the synchronous mode and the asynchronousmode.

FIG. 17A shows a method for generating a sync sequence used for aprimary sync channel in an asynchronous W-CDMA system. The sync sequenceis generated by XORing a hierarchical sequence H and a Walsh functionW_(o) on a chip unit basis.

FIG. 17B shows a scheme for generating a sync sequence for the syncchannel proposed in this embodiment of the present invention, whereinthe syn sequence of the sync channel for the synchronous system isdesigned to be orthogonal with a sync sequence of the sync channel forthe asynchronous system. This is to minimize a correlation value betweenthe sync sequences used in the different systems. The sync sequence forthe sync channel in the embodiment shown in FIG. 17B is generated byXORing the hierarchical sequence H used in the asynchronous mode and theWalsh function W_(n) on a chip unit basis. The Walsh function W_(n) isselected from the Walsh functions which are not used in the asynchronousmode.

When the base station system operates in the synchronous mode, the basestations can be identified using different spreading codes or the PNoffsets of the spreading code. The number of the PN spreading codes usedfor the forward link can be 1 or more. When the number of the PN code isone and the number of the PN offsets given for the PN code isNUM_OFFSET1, it is possible to distinguish NUM_OFFSET1 different cells.When one PN code is used, to have the number of distinguishable cellsbecome equal as compared with a case where NUM_PN PN codes are used, itis necessary to increase the period of the one PN spreading code used ascompared with a case where several PN codes are used.

Therefore, when one PN code is used, it is necessary to use a PN code ofa longer period or a PN offset of a shorter length as compared with thecase where NUM_PN PN codes are used. The present invention will bedescribed with reference to an embodiment wherein 16 different spreadingcodes are used in the synchronous mode and 32 different PN offsets areapplied to each spreading code so as to make it possible to distinguish512 base stations. The reason for combining several spreading codes andPN offsets unlike the IS-95 system is to apply the invention not only toa case where the base stations are exactly time synchronized to eachother using the GPS, but also to a case where the base stations areroughly time synchronized using the system network. That is, when thebase stations acquire time synchronization using the network, it isdifficult to acquire an exact time synchronization as in the case wherethe GPS is used. However, in the IS-95 system, since a unit PN offset isabout 50 μsec, it is difficult to acquire such synchronization usingnetwork synchronization. Therefore, in order to increase an intervalbetween the offsets, it is necessary either to increase the length ofthe spreading code or to use an increased number of spreading codes.

When a cell operating in the synchronous mode is adjacent to a celloperating in the asynchronous mode and the two cells use the same PNcode, there may not be guaranteed a PN offset required fordistinguishing a PN code for the cell operating in the synchronous modefrom a PN code for the cell operating in the asynchronous mode.Therefore, the PN codes used in the synchronous mode should be differentfrom the PN codes used in the asynchronous mode. A PN spreading codeused in the synchronous mode base station should be different from a PNspreading code used in the asynchronous mode base station. To this end,an embodiment of the present invention uses new PN spreading codes whichare different in number from the 512 PN spreading codes used in theasynchronous mode. In this embodiment of the present invention, 16 newPN spreading codes are assigned for the synchronous mode base stations.

FIG. 18 shows operation of a mobile station in the case where a basestation may operate in either the synchronous mode or asynchronous mode.In the embodiment of FIG. 18, the mobile station should first determinein which mode the base station, to which it belongs, operates. First, instep 1813, the mobile station determines whether acquisition isperformed for the synchronous mode or asynchronous mode in a systemselection step. When it is determined that the mobile station acquiresthe asynchronous mode, the mobile station performs the conventionalthree-step initial cell search process. In this process, the mobilestation searches for slot synchronization in a first step (1815);performs code group selection and frame synchronization in a second step(1817); and selects a base station code from the code group in a thirdstep (1819). On the contrary, when acquisition for the synchronous modeis selected, the mobile station searches frame synchronization in afirst step (1814), and determines a base station code in a second step(1818).

The network transmits neighbor cell list information to the mobilestation through a broadcasting channel (BCH) or a forward common channelduring handoff, idle mode search or active mode search. FIGS. 19A to 19Cshow a 10-bit data field for expressing the neighbor cell list. When thebase station operates in the synchronous mode or asynchronous mode,information represented by each bit of the data field can be defineddifferently.

FIG. 19A shows a data field format of the neighbor cell list for asystem operating in the asynchronous mode. Since the system operating inthe asynchronous mode uses 512 different base station codes, theneighbor cell list field can be defined as follows. A 1st bit indicateswhether the system operates in the synchronous mode or asynchronousmode. 2nd to 6th bits indicate which code group is used out of the 32code groups. 7th to 10th bits indicate which code is used out of the 16base station codes in each code group.

FIG. 19B shows a data field format of the neighbor cell list for thesynchronous mode where the cells are identified using one spreading codeand several PN offsets. A 1st bit indicates whether the system operatesin the synchronous mode or asynchronous mode. 2nd to 10th bits indicatewhich PN offset is used out of 512 PN offsets of the single spreadingcode.

FIG. 19C shows a data field format of the neighbor cell list for thesynchronous mode where the cells are identified using several spreadingcodes and PN offsets. A 1st bit indicates whether the system operates inthe synchronous mode or asynchronous mode. 2nd to 6th bits indicatewhich PN offset is used out of 32 PN offsets for each spreading code.7th to 10th bits indicate which code is used out of 16 base stationcodes belonging to the synchronous mode. If the number of the used basestation codes and the number of the PN offsets for each code arechanged, the length of the corresponding fields may be changed.

In the synchronous mode or operation, whether to operate as shown inFIG. 19B or 19C is previously determined between the base station andthe mobile station.

As described above, the novel CDMA communication system can effectivelyperform transmission of the sync channels and sync acquisition withinone period of the spreading code. Further, the asynchronous W-CDMAcommunication system can perform communication in a synchronous modeusing a single sync channel. Therefore, the novel synchronization methodcan minimize interference on a forward link by reducing the frequency ofsync channel transmissions, thereby increasing the system capacity.

While the invention has been shown and described with reference tocertain preferred embodiments thereof, it will be understood by thoseskilled in the art that various changes in form and details may be madetherein without departing from the spirit and scope of the invention asdefined by the appended claims.

1. A synchronization (sync) code transmission device for a base stationin a CDMA (Code Division Multiple Access) communication system,comprising: a primary sync code transmitter for generating a primarysync code, said primary sync code indicating a starting point of oneframe, said frame being equal to one period of a spreading code, and fortransmitting the primary sync code at a first location in the frame; anda secondary sync code transmitter for generating a secondary sync codeassigned to a group of base stations including the base station, and fortransmitting the secondary sync code at a second location in the frame.2. The sync code transmission device as claimed in claim 1, wherein theprimary sync code is a common code used by every base station and thesecondary sync code is a code for identifying the group of the basestations.
 3. The sync code transmission device as claimed in claim 2,further comprising a forward common channel signal transmitter fortransmitting a spreading code which is specific to a base station and isused for indicating a base station.
 4. The sync code transmission deviceas claimed in claim 3, wherein the forward common channel signaltransmitter is a pilot channel signal transmitter.
 5. The sync codetransmission device as claimed in claim 3, wherein the forward commonchannel signal transmitter is a broadcasting channel signal transmitter.6. The sync code transmission device as claimed in claim 2, wherein theprimary sync code and the secondary sync code are transmitted at leastonce within one period of the spreading code.
 7. The sync codetransmission device as claimed in claim 1, wherein the first location isa starting point of one frame.
 8. The sync code transmission device asclaimed in claim 1, wherein the first location is an ending point of oneframe.
 9. The sync code transmission device as claimed in claim 1,wherein the first location is a location separated from a starting pointof one frame by a predetermined chip length.
 10. The sync codetransmission device as claimed in claim 1, wherein a time intervalbetween the first location and the second location is such that ademodulator in a mobile station can perform coherent demodulation. 11.The sync code transmission device as claimed in claim 1, wherein thesecondary sync code is information for indicating a code group to whichthe base station belongs.
 12. A synchronization (sync) code transmissiondevice for a base station in a CDMA (Code Division Multiple Access)communication system, comprising: a primary sync code transmitter forgenerating a primary sync code, said primary sync code for indicating astarting point of a frame, said frame being equal to one period of aspreading code, and for transmitting at least one said primary sync codeat a specific location in the frame; and a secondary sync codetransmitter for generating a secondary sync code assigned to a group towhich the base station belongs, and for transmitting the secondary synccode at a time slot alter transmission of the primary sync code, saidtime slot being assigned to at least one base station group, said framehaving at least two time slots.
 13. The sync channel signal transmissiondevice as claimed in claim 12, wherein each of the at least two timeslots in said frame are assigned to a plurality of base station groupand the secondary sync codes are codes for indicating a base stationgroup out of the plurality of base station groups assigned to a specifictime slot.
 14. The sync code transmission device as claimed in claim 13,wherein secondary sync codes assigned to a time slot are orthogonal toeach other.
 15. The sync code transmission device as claimed in claim13, wherein a guard interval between time slots has a specific chipsize.
 16. A synchronization (sync) code transmission device for a basestation in a CDMA (Code Division Multiple Access) communication system,comprising: a primary sync code transmitter for generating a primarysync code, said primary sync code for indicating a starting point of aframe, said frame having one period of a spreading code, and fortransmitting the primary sync code at a first and third locations in theframe; a secondary sync code transmitter for generating a secondary synccode, said secondary sync code assigned to a group of base stations,said group including the base station, and for transmitting thesecondary sync code at a second and fourth locations in the frame; andan antenna diversity system having at least two antennas comprising: afirst antenna for transmitting the primary and secondary sync codes atthe first and third locations; and a second antenna transmitting theprimary and secondary sync codes at the second and fourth location. 17.The sync code transmission device as claimed in claim 16, wherein theprimary sync code is a common code used by every base station, and thesecondary sync code is a code for identifying a group of base stations.18. The sync code transmission device as claimed in claim 16, whereinthe first location and the third location are located a ½ frame periodapart.
 19. The sync code transmission device as claimed in claim 16,wherein the first location is a starting point of the frame.
 20. Asynchronization (sync) code transmission method for a base station in aCDMA (Code Division Multiple Access) communication system, comprisingthe steps of: generating a primary sync code, said primary sync code forindicating a starting point of a frame, said frame having one period ofa spreading code; transmitting the primary sync code at a first locationin the frame; generating a secondary sync code, said secondary codeassigned to a group of base stations including the base station; andtransmitting the secondary sync code at a second location in the frame.21. A synchronization (sync) code transmission method for a base stationin a CDMA (Code Division Multiple Access) communication system wherein aframe has at least two time slots and each time slot is assigned to aplurality of base stations, the method comprising the steps of:generating a primary sync code, said primary sync code for indicatingsynchronization at a starting point of a frame, said frame having oneperiod of a spreading code; transmitting at least one said primary synccode at a specific location in the frame; generating a secondary synccode, said secondary sync code assigned to a group to which the basestation belongs; and transmitting the secondary sync code at a time slotassigned to the corresponding base station group after transmission ofthe primary sync code.
 22. A synchronization (sync) code transmissionmethod for a base station in a CDMA (Code Division Multiple Access)communication system supporting a transmit diversity function, saidcommunication system having at least two antennas, a sync code generatorfor generating a primary sync code, said primary sync code forindicating synchronization at a starting point of a frame, said framehaving one period of a spreading code, said sync code generator also forgenerating a secondary sync code, said secondary sync code assigned to agroup of base stations including the base station, the method comprisingthe steps of: transmitting the primary sync code at a first location ofthe frame through a first antenna; transmitting the secondary sync codeat a second location of the frame through a first antenna; transmittingthe primary sync code at a third location of the frame through a secondantenna; and transmitting the secondary sync code at a fourth locationof the frame through a second antenna.
 23. A synchronization (sync) codereceiving device for a mobile station in a CDMA (Code Division MultipleAccess) communication system, comprising: a primary sync codeacquisition decider for acquiring a primary sync code received at afirst location in a frame, and for acquiring synchronization at astarting point of a frame, said frame equal to one period of a spreadingcode; and a base station group decider for, once enabled uponacquisition of the primary sync code, receiving a secondary sync codetransmitted at a second location in the frame, and for deciding a basestation group to which the transmitting base station belongs.
 24. Asynchronization (sync) code receiving device for a mobile station in aCDMA (Code Division Multiple Access) communication system wherein aframe has at least two time slots, and each of said at least two timeslots is assigned to a plurality of base station groups, the devicecomprising: a primary sync code acquisition decider for acquiring aprimary sync code received at a first location in a frame, and foracquiring synchronization at a starting point of a frame, said framebeing equal to one period of a spreading code; and a base station groupdecider for, once enabled upon acquisition of the primary sync code,performing despreading with secondary sync codes, said secondary synccodes being of the assigned base station groups at each time slot, andfor deciding which base station group corresponds to a secondary synccode having a highest value out of the despread signals.
 25. Asynchronization (sync) code receiving device for a mobile station in aCDMA (Code Division Multiple Access) communication system, said systemhaving at least one base station, at least one base station having atleast two antennas to support a transmit diversity function, the devicecomprising: a primary sync code acquisition decider for acquiring aprimary sync code received at first and third locations in a frame, andfor acquiring synchronization at a starting point of a frame, said framebeing equal to one period of a spreading code; and a base station groupdecider for, once being enabled upon acquisition of the primary synccode, performing despreading with secondary sync codes at second andfourth locations in the frame, each of said secondary sync codescorresponding to a base station group, and for deciding which basestation group corresponds to a secondary sync code having a highestvalue out of the despread signals.
 26. A synchronization (sync) codereceiving method for a mobile station in a CDMA (Code Division MultipleAccess) communication system, comprising the steps of: receiving aprimary sync code transmitted at a first location in a frame; acquiringsynchronization at a starting point of a frame, said frame being equalto one period of a spreading code; receiving a secondary sync codetransmitted at a second location in the frame; and deciding a basestation group to which a transmitting base station belongs.
 27. Asynchronization (sync) code receiving method for a mobile station in aCDMA (Code Division Multiple Access) communication system, said systemusing transmission frames, a frame having at least two time slots, andeach time slot being assigned to a plurality of base station groups, themethod comprising the steps of: receiving a primary sync code at a firstlocation in a frame; acquiring synchronization at a starting point of aframe, said frame having one period of a spreading code; performingdespreading with secondary sync codes, each of said secondary sync codesbeing assigned to a base station group and a time slot; and deciding abase station group corresponding to a secondary sync code having ahighest value out of the despread signals.
 28. A synchronization (sync)code communication device in a CDMA (Code Division Multiple Access)communication system, comprising: a base station comprising: a primarysync code transmitter for generating a primary sync code, said primarysync code for acquiring synchronization at a starting point of a frame,said frame being equal to one period of a spreading code, and fortransmitting the primary sync code at a first location in the frame; asecondary sync code transmitter for generating a secondary sync code,said secondary sync code being assigned to a group of base stationsincluding the base station, and for transmitting the secondary sync codeat a second location in the frame; a mobile station comprising: aprimary sync code acquisition decider for acquiring a primary sync codereceived at a first location in a frame, and for acquiringsynchronization at a starting point of a frame, said frame being equalto one period of a spreading code; and a base station group decider for,once being enabled upon acquisition of the primary sync code, receivinga secondary sync code transmitted at a second location in the frame, andfor deciding a base station group to which the corresponding basestation belongs.
 29. A synchronization (sync) code communication devicein a CDMA (Code Division Multiple Access) communication system,comprising: a base station comprising: a primary sync code transmitterfor generating a primary sync code, said primary sync code for acquiringsynchronization at a starting point of a frame, said frame being equalto one period of a spreading code, and for transmitting said primarysync code at least one specific location in the frame; a secondary synccode transmitter for generating a secondary sync code, said secondarysync code being assigned to a base station group to which the basestation belongs, and for transmitting the secondary sync code at a timeslot after transmission of the primary sync code, said time slotassigned to the base station group, said frame having at least two timeslots; a mobile station including; a primary sync code acquisitiondecider for acquiring a primary sync code received at a first locationin a frame, and for acquiring synchronization at a starting point of aframe, said frame being equal to one period of a spreading code; and abase station group decider for, once being enabled upon acquisition ofthe primary sync code, performing despreading with secondary sync codesof assigned base station groups at each time slot, and for deciding abase station group corresponding to a secondary sync code having ahighest value out of the despread signals.
 30. A synchronization (sync)signal transmission method in a communication system, the methodcomprising: transmitting, by a primary sync signal transmitter, aprimary sync signal at every transmission interval; and transmitting, bya secondary sync signal transmitter, a secondary sync signal at everytransmission interval, wherein the transmission interval (P) is at P=1and at P=T/2+1, where T is a total number of slots in a frame and T>2,the primary sync signal transmitted at P=1 is in a first location of theframe, and the primary sync signal transmitted at P=T/2+1 is in a secondlocation of the frame, and the second sync signal transmitted at P=1 isin a third location of the frame, and the second sync signal transmittedat P=T/2+1 is in a fourth location of the frame, the third and fourthlocations being different from the first and second locations in theframe, and wherein the first location at P=1 is offset by apredetermined time from a starting point of the frame, and the secondlocation at P=T/2+1 is offset by the predetermined time from a halfpoint of a total length of the frame.
 31. The sync signal transmissionmethod as claimed in claim 30, wherein the third location at P=1 isoffset by a predetermined time from the starting point of the frame, andthe fourth location at P=T/2+1 is offset by a predetermined time fromthe half point of the total frame length.
 32. The sync signaltransmission method as claimed in claim 30, wherein the primary andsecondary sync signals comprise one symbol in time domain.
 33. The syncsignal transmission method as claimed in claim 30, wherein the secondarysync signals are generated according to transmission durationinformation.
 34. The sync signal transmission method as claimed in claim30, wherein the primary and secondary sync signals are transmitted onconsecutive symbols in time domain.
 35. The sync signal transmissionmethod as claimed in claim 30, wherein the secondary sync signals at thethird and fourth locations uniquely specify a cell group or frame sync.36. A synchronization (sync) signal receiving method in a communicationsystem, the method comprising: receiving, by a primary sync signalreceiver, a primary sync signal at every transmission interval; andreceiving, by a secondary sync signal receiver, a secondary sync signalat every transmission interval, wherein the transmission interval (P) isat P=1 and at P=T/2+1, where T is a total number of slots in a frame andT>2, the primary sync signal transmitted at P=1 is in a first locationof the frame, and the primary sync signal transmitted at P=T/2+1 is in asecond location of the frame, and the second sync signal transmitted atP=1 is in a third location of the frame, and the second sync signaltransmitted at P=T/2+1 is in a fourth location of the frame, the thirdand fourth locations being different from the first and second locationsin the frame, and wherein the first location at P=1 is offset by apredetermined time from a starting point of the frame, and the secondlocation at P=T/2+1 is offset by the predetermined time from a halfpoint of a total length of the frame.
 37. The sync signal receivingmethod as claimed in claim 36, wherein the third location at P=1 isoffset by a predetermined time from the starting point of the frame, andthe fourth location at P=T/2+1 is offset by a predetermined time fromthe half point of the total frame length.
 38. The sync signal receivingmethod as claimed in claim 36, wherein the primary and secondary syncsignals comprise one symbol in time domain.
 39. The sync signalreceiving method as claimed in claim 36, wherein the secondary syncsignals are generated according to transmission duration information.40. The sync signal receiving method as claimed in claim 36, wherein theprimary and secondary sync signals are received on consecutive symbolsin time domain.
 41. The sync signal receiving method as claimed in claim36, wherein the secondary sync signals at the third and fourth locationsuniquely specify a cell group or frame sync.
 42. A synchronization(sync) signal transmission apparatus in a communication system, theapparatus comprising: a primary sync signal transmitter for transmittinga primary sync signal at every transmission interval; and a secondarysync signal transmitter for transmitting a secondary sync signal atevery transmission interval, wherein the transmission interval (P) is atP=1 and at P=T/2+1, where T is a total number of slots in a frame andT>2, the primary sync signal transmitted at P=1 is in a first locationof the frame, and the primary sync signal transmitted at P=T/2+1 is in asecond location of the frame, and the second sync signal transmitted atP=1 is in a third location of the frame, and the second sync signaltransmitted at P=T/2+1 is in a fourth location of the frame, the thirdand fourth locations being different from the first and second locationsin the frame, and wherein the first location at P=1 is offset by apredetermined time from a starting point of the frame, and the secondlocation at P=T/2+1 is offset by the predetermined time from a halfpoint of a total length of the frame.
 43. The sync signal transmissionapparatus as claimed in claim 42, wherein the third location at P=1 isoffset by a predetermined time from the starting point of the frame, andthe fourth location at P=T/2+1 is offset by a predetermined time fromthe half point of the total frame length.
 44. The sync signaltransmission apparatus as claimed in claim 42, wherein the primary andsecondary sync signals comprise one symbol in time domain.
 45. The syncsignal transmission apparatus as claimed in claim 42, wherein thesecondary sync signals are generated according to transmission durationinformation.
 46. The sync signal transmission apparatus as claimed inclaim 42, wherein the primary and secondary sync signals are transmittedon consecutive symbols in time domain.
 47. The sync signal transmissionapparatus as claimed in claim 42, wherein the secondary sync signals atthe third and fourth locations uniquely specify a cell group or framesync.
 48. A synchronization (sync) signal receiving apparatus in acommunication system, the apparatus comprising: a primary sync signalreceiver for receiving a primary sync signal at every transmissioninterval; a secondary sync signal receiver for receiving a secondarysync signal at every transmission interval; and a controller foracquiring a frame sync and a cell identification from the primary syncsignal and the secondary sync signal, wherein the transmission interval(P) is at P=1 and at P=T/2+1, where T is a total number of slots in theframe and T>2, the primary sync signal transmitted at P=1 is in a firstlocation of a frame, and the primary sync signal transmitted at P=T/2+1is in a second location of the frame, and the second sync signaltransmitted at P=1 is in a third location of the frame, and the secondsync signal transmitted at P=T/2+1 is in a fourth location of the frame,the third and fourth locations being different from the first and secondlocations in the frame, wherein the first location at P=1 is offset by apredetermined time from a starting point of the frame, and the secondlocation at P=T/2+1 is offset by the predetermined time from a halfpoint of a total length of the frame.
 49. The sync signal receivingapparatus as claimed in claim 48, wherein the third location at P=1 isoffset by a predetermined time from the starting point of the frame, andthe fourth location at P=T/2+1 is offset by a predetermined time fromthe half point of the total frame length.
 50. The sync signal receivingapparatus as claimed in claim 48, wherein the primary and secondary syncsignals comprise one symbol in time domain.
 51. The sync signalreceiving apparatus as claimed in claim 48, wherein the secondary syncsignals are generated according to transmission duration information.52. The sync signal receiving apparatus as claimed in claim 48, whereinthe primary and secondary sync signals are received on consecutivesymbols in time domain.
 53. The sync signal receiving apparatus asclaimed in claim 48, wherein the secondary sync signals at the third andfourth locations uniquely specify a cell group or the frame sync.